Walking Fish Sheds Light On The Evolution Of Vertebrate Limbs

From time to time, long, wriggly fish called bichirs enjoy walks along the banks of the Nile with its robust fins, breathing fresh air with its functional lungs. By raising this primitive fish on land, researchers reveal how our fishy ancestors may have left the oceans behind. The findings, published in Nature this week, suggest that the ability of a developing animal to modify its behavior in response to change -- called developmental plasticity -- helped facilitate this key transition onto land.

You’ve seen it before on t-shirts and bumper stickers: A fish, amphibian, small mammal, monkey, and human all lined up in a row, with some variation and a punchline. A group of fish ancestors took that first step around 400 million years ago -- a move accompanied by the development of terrestrial locomotion and evolution of supportive limbs. Those ancient fish gave rise to the earliest four-legged vertebrates, or tetrapods.
The bichir (Polypterus senegalus) is a freshwater, ray-finned fish that can be found in tropical east Africa today. Out of water, it walks like a tetrapod: Placing the pectoral fins forward, the fish pushes its head and the front of the body off the ground, while the tail and body work together to push the rest of it forward over the fin. It’s a living analog for “terrestrialization.”

Photo credit: Polypterus senegalus walks by using its fins and body for support. Fins are placed forward and used to push the head and anterior body off the ground while the tail and body push the fish forward over the fin / Antoine Morin

So, Emily Standen from University of Ottawa and Trina Du and Hans Larsson from McGill University raised over a hundred 2-month-old bichirs on land in moist tanks for eight months and examined their adaptability to this new, terrestrial habitat. “Stressful environmental conditions can often reveal otherwise cryptic anatomical and behavioral variation, a form of developmental plasticity,” Standen explains in a news release. “We wanted to use this mechanism to see what new anatomies and behaviors we could trigger in these fish and see if they match what we know of the fossil record.”

The land-reared fish learned to walk more effectively. These bichirs lift their head higher off the ground, hold their fins closer to their body, and would slip less than bichirs raised in aquariums.

The sophisticated walkers also displayed muscle and bone structure changes. “Their pectoral skeleton changed to become more elongate with stronger attachments across their chest, possibly to increase support during walking,” Du says, “and a reduced contact with the skull to potentially allow greater head/neck motion.”

The study suggests that the plasticity of ancient fish provided the variation need for the evolution of terrestrially functional fins that eventually evolved into limbs for walking. “Because many of the anatomical changes mirror the fossil record, we can hypothesize that the behavioral changes we see also reflect what may have occurred when fossil fish first walked with their fins on land,” Larsson explains. These new anatomies and behaviors would later be genetically fixed by natural selection.
Watch bichirs walking in this delightful video: